1. Field of the Invention
The present invention relates to power converters, and more particularly, relates to a switching controller to reduce acoustic noise of power converters.
2. Description of the Related Art
FIG. 1 shows a schematic circuit of a conventional power converter. For safety reason, the power converter generally includes a transformer 10 having a primary winding NP and a secondary winding NS for providing galvanic isolation. An input voltage VIN is supplied to a first terminal of the primary winding VP of the transformer 10. A transistor 20 coupled to a second terminal of the primary winding VP of the transformer 10 is controlled by a switching controller 100. Once the transistor 20 is turned on, a switching current IP flowing through the transistor 20 will increase and the energy will be stored in the transformer 10. As the transistor 20 is turned off, the energy will be transmitted to the secondary winding NS of the transformer 10. An output voltage VO built across a capacitor 45 is converted from the energy stored in the transformer 10 via a rectifier 40. Once the output voltage VO exceeds a reverse breakdown voltage of a zenor diode 50, a feedback voltage VFB indicative of the output voltage VO of the power converter will be provided to the switching controller 100 via a photo-coupler 60. The zenor diode 50 and a resistor 51 are coupled in series between the rectifier 40 and the photo-coupler 60. A current-sense resistor 30 is coupled between the transistor 20 and a ground reference for converting the switching current IP into a switching-current signal VCS. Therefore, the switching-current signal VCS is in proportion to the switching current IP. The switching controller 100 generates a switching signal SOUT to control the transistor 20 in response to the switching-current signal VCS and the feedback voltage VFB.
Referring to FIG. 1, the power converter further comprises a resistor 26 and a capacitor 24 which are coupled in series between the first terminal of the primary winding NP of the transformer 10 and the ground reference. The transformer 10 further comprises an auxiliary winding NA. The auxiliary winding NA is magnetically coupled to the primary winding NP. A rectifier 22 is coupled between a joint of the resistor 26 and the capacitor 24 and a first terminal of the auxiliary winding NA. A second terminal of the winding NA is coupled to the ground reference.
Following equation shows a magnetic force F of the transformer 10:F=B×l×N×IP 
where B is the flux density; l is the conductor length; N is the winding-turns; and IP is the switching current flowing through the transformer 10.
The magnetic force F produced at the air-gap of the transformer 10 may generate acoustic noise if a switching frequency of the switching current IP is in the audio band. According to the equation above, limiting the switching current IP decreases the magnetic force F of the transformer 10 and the acoustic noise of the power converter can be therefore reduced.